During the fabrication of semiconductor devices for integrated circuits (ICs), a sequence of material processing steps, including both pattern etching and deposition processes, are performed, whereby material is removed from or added to a substrate surface, respectively. During, for instance, pattern etching, a pattern formed in a mask layer. of radiation-sensitive material, such as photoresist, using for example photolithography, is transferred to an underlying thin material film using a combination of physical and chemical processes to facilitate the selective removal of the underlying material film relative to the mask layer.
Thereafter, the remaining radiation-sensitive material, or photoresist, and post-etch residue, such as hardened photoresist and other etch residues, are removed using one or more cleaning processes. Conventionally, these residues are removed by performing plasma ashing in an oxygen plasma, followed by wet cleaning through immersion of the substrate in a liquid bath of stripper chemicals.
Until recently, dry plasma ashing and wet cleaning were found to be sufficient for removing residue and contaminants accumulated during semiconductor processing. However, recent advancements for ICs include a reduction in the critical dimension for etched features below a feature dimension acceptable for wet cleaning, such as a feature dimension below approximately 45 to 65 nanometers (nm). Moreover, the advent of new materials, such as low dielectric constant (low-k) materials, limits the use of plasma ashing due to their susceptibility to damage during plasma exposure.
At present, interest has developed for the replacement of dry plasma ashing and wet cleaning. One interest includes the development of dry cleaning systems utilizing a supercritical fluid as a carrier for a solvent, or other residue removing composition. The use of supercritical carbon dioxide, for example, in processing semiconductor wafers has been shown in the art.
Certain challenges occur when attempting to process silicon wafers under high pressure. One such issue is how to hold the wafer in place during processing. It has been shown that a wafer can be supported at discrete locations around its edge, with high pressure supercritical carbon dioxide (SCCO2) surrounding the entire wafer.
A different approach is to hold the wafer down on a platen using vacuum or reduced pressure from the top surface of the wafer, which has also been shown. In such a case, bias in pressure keeps the wafer in place during processing, which may include violent events such as sudden decompressions, high surface velocity jets for cleaning, etc. One of the significant drawbacks of vacuum holding is the restraining of the wafer against the platen. With such a large surface area of 300 millimeter (mm) wafers, for example, the exposed area of the platen, is subjected to loads that can exceed half a million pounds. Even very thick steels platens will deflect under this kind of load. Typical pressures encountered in SCCO2 processing are a minimum of 1,031 psi, but 3,000 psi is not uncommon, and upwards of 10,000 psi has been reported in the literature.
If a wafer is held against a platen, typically of stainless steel, the resulting static pressure load can force the wafer against the platen, which can cause damage to the backside of the wafer. Particulates that may be present can then get embedded into the platen or into the backside of the wafer. This can cause irreparable harm to the wafer for subsequent process steps.
Another effect of these high forces is the flexing of the platen under the pressure load. As the pressure increases, the wafer becomes restrained against the platen. As pressure continues to increase, the platen can bow due to the load. The wafer may or may not be able to follow the new shape that the platen is forced into due to the pressure load. Once the pressure is released or reduced, the wafer must again readjust for the change in shape of the platen. If multiple pressure cycles are applied, this effect can be repeated many times on a single wafer.
Results of this flexing can break wafers, because they are brittle and fragile and cannot elastically deform like stainless steel. It can also cause a grinding or fretting effect between the wafer and the platen, due to the high forces and small displacements which take place. This can create metal or silicon particles to be interspersed between the wafer and platen, which in turn can damage the current wafer, and be present on the platen to damage subsequent wafers that are processed.
The magnitude of this flexing may be considered trivial under ordinary industrial circumstances. Unfortunately with semiconductor wafers, flexing of less than 0.0010 inches, or even as little as 0.0005 inches, have been shown to cause significant damage to wafers, or wafer breakage.
At present, the inventors have recognized that if the force holding the wafer to the platen is reduced, the wafer can slip in relation to the platen, and the likelihood of breakage can be reduced. If the holding force is reduced even further in magnitude, then wear can also be eliminated because there would not be enough frictional force to create wear or particles.
Misalignment of features on the wafer platen, or poor flatness of the platen surface can also result in wafer breakage if the holding load is high. If the wafer is required to span over holes or slots in the platen, then the wafer becomes a “bridge” with the entire pressure load bearing down on an unsupported region of thin silicon. It doesn't take a very large span to break a wafer when subjected to 3,000 psi or higher pressures.
Accordingly, there is a need to overcome the above described problems.